Their industry already upended with the passage of the federal health care law, insurance companies are facing another upheaval if the Supreme Court rules that millions of Americans are not eligible for subsidies to help defray the cost of their coverage.

The court is expected to decide by the end of June or in early July whether it agrees with the plaintiffs in King v. Burwell that the language in the Affordable Care Act allows the government to offer subsidies only in those states that have established their own insurance marketplaces.